catalysts

Article

Low-Temperature Oxidation of Dimethyl Ether toPolyoxymethylene Dimethyl Ethers overCNT-Supported Rhenium Catalyst

Qingde Zhang 1, Wenfeng Wang 1,2, Zhenzhou Zhang 1,2, Yizhuo Han 1,* and Yisheng Tan 1,*1 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,

Taiyuan 030001, China; qdzhang@sxicc.ac.cn (Q.Z.); wangwf@sxicc.ac.cn (W.W.); zhangzz@sxicc.ac.cn (Z.Z.)2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China* Correspondence: hanyz@sxicc.ac.cn (Y.H.); tan@sxicc.ac.cn (Y.T.); Tel./Fax: +86-351-404-4287 (Y.T. & Y.H.);

+86-351-404-9747 (Y.T. & Y.H.)

Academic Editor: Stuart H. TaylorReceived: 14 December 2015; Accepted: 3 March 2016; Published: 14 March 2016

Abstract: Due to its excellent conductivity, good thermal stability and large specific surface area,carbon nano-tubes (CNTs) were selected as support to prepare a Re-based catalyst for dimethyl ether(DME) direct oxidation to polyoxymethylene dimethyl ethers (DMMx). The catalyst performance wastested in a continuous flow type fixed-bed reactor. H3PW12O40 (PW12) was used to modify Re/CNTsto improve its activity and selectivity. The effects of PW12 content, reaction temperature, gas hourlyspace velocity (GHSV) and reaction time on DME oxidation to DMMx were investigated. The resultsshowed that modification of CNT-supported Re with 30% PW12 significantly increased the selectivityof DMM and DMM2 up to 59.0% from 6.6% with a DME conversion of 8.9%; besides that, there wasno COx production observed in the reaction under the optimum conditions of 513 K and 1800 h´1.The techniques of XRD, BET, NH3-TPD, H2-TPR, XPS, TEM and SEM were used to characterize thestructure, surface properties and morphology of the catalysts. The optimum amount of weak acidsites and redox sites promotes the synthesis of DMM and DMM2 from DME direct oxidation.

Keywords: dimethyl ether; low-temperature oxidation; polyoxymethylene dimethyl ethers; carbonnano-tubes; Re; H3PW12O40

1. Introduction

Dimethyl ether (CH3OCH3, DME) is a clean fuel with high cetane number and is also a potentialand non-petroleum route chemical synthesis material. DME can be synthesized via one-step processat low cost from syngas generated from coal, biomass and natural gas. Because of the low boilingpoint of DME (246.3 K), it is not possible to simply replace diesel with DME or directly blend DMEwith diesel. Polyoxymethylene dimethyl ethers (CH3O(CH2O)xCH3, DMMx) are promising diesel oiladditives due to the similar structure of –C–O–C–O–C–O–C– with DME and high content of oxygenand cetane number [1]. The addition of DMMx to diesel oil can greatly improve the combustionand reduce particular matter emissions of diesel engines. In indusry, DMMx is mainly produced viacondensation of methanol and trioxymethylene over acidity catalysts [2], but this synthesis technologyhas the problems of high energy consumption, high investment and high operation cost. Utilizingoxidation of DME to synthesize DMMx is one of the most attractive green routes for the synthesis ofclean fuel additives with a short process, low CO2 emissions and high energy efficiency.

DME oxidation has been paid more and more attention due to the advantages of simplicity andfeasibility [3–8]. Yagita Hirosh et al. examined DME oxidation to 1,2-dimethoxyethane (DMET) overa SnO2/MgO catalyst [9]. Wenjie Shen et al. investigated the supported MoOx and VOx catalysts

Catalysts 2016, 6, 43; doi:10.3390/catal6030043 www.mdpi.com/journal/catalysts

Catalysts 2016, 6, 43 2 of 12

for DME oxidation to HCHO [10]. Haichao Liu et al. reported that the synthesis of DMM from theoxidation of DME and methanol using H3+nVnMo12´nPO40 [11]. In recent years, our group has beenfocusing on the selective oxidation of DME to HCHO, methyl formate (MF), DMMx, etc. over differentcatalysts [12–15].

In our previous work, a Mn-(Sm+SiW12)/SiO2 catalyst exhibited good activity for the selectiveoxidation of DME to DMM at 593K, but by-product COx was usually formed due to a high reactiontemperature [16]. Though the DMM synthesis from DME oxidation has been realized, furtherenhancing the chain growth of C–O to obtain larger DMMx molecules from DME direct oxidation atlow temperature is still a very challenging task.

Rhenium oxide is widely used in some oxidation reactions due to its unique redox and acidicproperties [17–23]. We have also found that Re/TiO2 was active for the selective oxidation of DMEand DMM as co-reactants to DMM2 [14]; however, the low surface area of TiO2 affects the dispersionof active components, and catalyst particles are prone to sintering during the reaction, which restrainsthe increase of catalyst activity. Carbon nano-tubes (CNTs) have been applied as support in catalyticreactions because of their excellent conductivity, good thermal stability and large specific surfacearea [24–28].

In the present study, CNTs were selected as support due to their unique surface properties.The H3PW12O40 (PW12), which can offer acidity, was used to modify Re/CNTs. The effects of PW12

content, reaction temperature, gas hourly space velocity (GHSV), and reaction time on DME oxidationto DMMx were investigated. The results show that the PW12-modified Re/CNTs demonstrateshigh activity and selectivity for the formation of DMM and DMM2 via DME direct oxidation at lowtemperature. The total selectivity of DMM and DMM2 reaches 59.0% with DME conversion of 8.9% at513 K without COx formation over 5%Re-30%PW12/CNTs. The techniques of XRD, BET, NH3-TPD,H2-TPR, XPS, TEM and SEM are used to characterize the structure, surface properties and morphologyof the catalysts. Until now, there have been no reports about the DME oxidation to DMM and DMM2

over CNT-supported Re catalyst.

2. Results and Discussion

2.1. Effects of PW12 Content on the Performance of 5%Re-PW12/CNTs

Table 1 shows the selectivity and the conversion of DME as a function of PW12 contentin 5%Re-PW12/CNTs. Over a CNT-supported Re catalyst, DME conversion is only 4.2%, andDMM selectivity is as low as 6.6% and no DMM2 is found, but HCHO selectivity reaches 73.7%,which indicates that Re/CNTs exhibits more redox sites than acid sites. When 5%PW12 is used tomodify Re/CNTs, there is an evident increase in DMM selectivity, and a trace of DMM2 is formed.After 20%PW12 introduction to Re/CNTs, DMM selectivity is clearly increased to 45.9%, and itsselectivity reaches 4.1%. The selectivity of DMM and DMM2 reaches the highest value of 59.0%, andDME conversion is also increased to 8.9% when Re/CNTs is modified by 30%PW12. However, a furtherincrease of PW12 content leads to a decline in the selectivity of DMM1´2. Especially, when PW12

content reaches as high as 80%, DME is mainly oxidized to CO with 64.4% selectivity. The activity andselectivity of pure 30%PW12 before Re addition has been also investigated and the DMM selectivity of32.7% is obtained; besides that, DMM2 selectivity reaches 13.9%, but by-product CO is found with theselectivity of 14.6%.

The results show that the addition of PW12 has obvious effects on DME conversion and DMMselectivity. It can be seen in Table 1 that no COx is formed in the reaction of DME to DMM and DMM2

when CNTs are used as support along with an optimum amount of PW12 introduction under theconditions of 513 K and 1800 h´1. This may be due to the special adsorption capacity and excellentconductivity of CNTs.

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Table 1. Effects of PW12 content on the performance of 5%Re-PW12/CNTs.

CatalystsDME

Conversion(%)

Selectivity(C-mol%)

DMM DMM2 CH3OH HCHO MF CO CH4 CO2

Re/CNTs 4.2 6.6 0 16.2 73.7 3.5 0 0 0Re-5%PW12/CNTs 4.9 26.3 0.3 2.7 67.7 3.0 0 0 0Re-20%PW12/CNTs 6.1 45.9 4.1 2.6 45.0 2.4 0 0 0Re-30%PW12/CNTs 8.9 55.0 4.0 4.2 31.4 5.4 0 0 0Re-40%PW12/CNTs 9.5 50.7 3.7 4.4 38.9 2.3 0 0 0Re-80%PW12/CNTs 15.0 27.5 1.7 3.5 2.1 0.8 64.4 0 0

30%PW12/CNTs 10.0 32.7 13.9 16.8 21.4 0.6 14.6 0 0

Reaction conditions: atmospheric pressure, 513 K, cat.: 1 mL, 15 min, 1800 h´1, nO2:nDME = 1:1.

In our previous work, a possible reaction mechanism of DME oxidation to DMM was proposedand DMM synthesis needed acid sites and redox sites [7,16]. According to the present reaction results,DMM can probably be formed by the acetalization reaction of methanol (formed by DME hydrolysisover acid sites) and HCHO (oxidized by CH3OH over redox sites) at low temperature. We alsosuggest that DMM may be the intermediate for the formation of DMM2 via DME oxidation [14].CH3OCH2OCH2OCH3 may be synthesized via CH3OCH2OCH2 group (obtained after the cleavage ofthe terminal C–H bond of DMM molecule over the redox sites) combining CH3O (formed over acidsites) under the cooperation of the acid sites and the redox sites of the catalyst. Therefore, optimumamount of the acid sites and redox sites of the catalyst is beneficial to the formation of DMM andDMM2 from DME oxidation.

2.2. Effects of Reaction Temperature on the Performance of 5%Re-30%PW12/CNTs

Table 2 shows the effects of reaction temperature on DME conversion and the selectivity of DMMand DMM2 over 5%Re-30%PW12/CNTs. With increasing reaction temperature, DME conversionkeeps an upward trend because DME molecule is easily activated at higher temperatures. At 493 K,the total selectivity of DMM and DMM2 is 19.8% with DME conversion of 6.6%, but the HCHOselectivity is as high as 62.0%. The selectivity of DMM and DMM2 reaches the highest value of 59.0%,and HCHO selectivity clearly decreases to 31.4% when temperature is increased to 513 K. Then, theselectivity of DMM and DMM2 decreases constantly with the increase in temperature. At 533 K,the selectivity of DMM and DMM2 decreases to 49.8%, and, concurrently, by-product CO appearsand its selectivity is 13.2%. CO selectivity reaches 21.9% when temperature is further increased to553 K. Lower temperature is not the optimum conditions for DME oxidation to DMM and DMM2, andHCHO is the main by-product. However, higher temperature easily leads to over-oxidation of DME toproduce more CO. Therefore, the optimum reaction temperature is 513 K for DME direct-oxidation toDMM and DMM2 with high DMM1´2 selectivity and low by-product selectivity.

Table 2. Effects of reaction temperature on the performance of 5%Re-30%PW12/CNTs.

ReactionTemperature

(K)

DMEConversion

(%)

Selectivity(C-mol%)

DMM DMM2 CH3OH HCHO MF CO CH4 CO2

493 6.6 17.0 2.8 16.5 62.0 1.7 0 0 0513 8.9 55.0 4.0 4.2 31.4 5.4 0 0 0533 10.9 45.7 4.1 4.1 31.1 1.8 13.2 0 0553 12.3 43.8 3.7 3.9 26.0 0.7 21.9 0 0

Reaction conditions: atmospheric pressure, cat.: 1 mL, 15 min, 1800 h´1, nO2:nDME = 1:1.

Catalysts 2016, 6, 43 4 of 12

2.3. Effects of Gas Hourly Space Velocity on the Performance of 5%Re-30%PW12/CNTs

The effects of GHSV on DME oxidation to DMM1´2 over 5%Re-PW12/CNTs are shown in Table 3.As can be seen in Table 3, CH3OH and HCHO are the main products when GHSV is lower, whilethe higher GHSV results in higher HCHO selectivity. When GHSV is 1200 h´1, CH3O´ from DMEdecomposition tends to adsorb on the acid sites of the catalyst, then CH3OH is formed and concurrentlyis partly oxidized to HCHO over redox sites, so CH3OH and HCHO formed as the main by-products.HCHO easily desorbs from the catalyst surface and has less opportunity to react with methanol to formDMM1´2 when GHSV is higher than 1800 h´1. It is proposed that HCHO may be the intermediate ofDMM and DMM2 formation via DME direct oxidation. At a GHSV of 1800 h´1, DMM1´2 selectivityreaches a maximum of 59.0%.

Table 3. Effects of GHSV on the performance of 5%Re-30%PW12/CNTs.

GHSV (h´1)DME

Conversion(%)

Selectivity(C-mol%)

DMM DMM2 CH3OH HCHO MF CO CH4 CO2

1200 9.8 28.3 2.0 28.2 34.2 7.3 0 0 01800 8.9 55.0 4.0 4.2 31.4 5.4 0 0 02400 5.0 35.7 5.1 8.8 50.4 0 0 0 03000 3.4 34.9 0.1 9.3 55.7 0 0 0 0

Reaction conditions: atmospheric pressure, 513 K, cat.: 1 mL, 15 min, nO2:nDME = 1:1.

2.4. Effects of Reaction Time on the Performance of 5%Re-30%PW12/CNTs

The effects of reaction time on the conversion of DME and the selectivities of the main productsover the 5%Re-30%PW12/CNTs catalyst were investigated. As can be seen in Figure 1, the totalselectivity of DMM and DMM2 reaches 59.0% at 15 min. There is a slight decrease from 59.0% to49.3% in the selectivity of DMM and DMM2, and DME conversion has no obvious changes during the300-min reaction. The 5%Re-30%PW12/CNTs catalyst exhibits high initial activity.

Catalysts 2016, 6, 43  4 of 12 

2.3. Effects of Gas Hourly Space Velocity on the Performance of 5%Re‐30%PW12/CNTs 

The effects of GHSV on DME oxidation to DMM1‐2 over 5%Re‐PW12/CNTs are shown in Table 3. 

As can be seen in Table 3, CH3OH and HCHO are the main products when GHSV is lower, while the 

higher  GHSV  results  in  higher HCHO  selectivity. When  GHSV  is  1200  h−1,  CH3O−  from  DME 

decomposition  tends  to  adsorb  on  the  acid  sites  of  the  catalyst,  then  CH3OH  is  formed  and 

concurrently is partly oxidized to HCHO over redox sites, so CH3OH and HCHO formed as the main 

by‐products. HCHO easily desorbs from the catalyst surface and has less opportunity to react with 

methanol to form DMM1‐2 when GHSV is higher than 1800 h−1. It is proposed that HCHO may be the 

intermediate of DMM and DMM2 formation via DME direct oxidation. At a GHSV of 1800 h−1, DMM1–2 

selectivity reaches a maximum of 59.0%. 

Table 3. Effects of GHSV on the performance of 5%Re‐30%PW12/CNTs. 

GHSV (h−1) DME Conversion 

(%) 

Selectivity 

(C‐mol%) 

DMM DMM2 CH3OH HCHO MF  CO  CH4  CO2

1200  9.8  28.3  2.0  28.2  34.2  7.3  0  0  0 

1800  8.9  55.0  4.0  4.2  31.4  5.4  0  0  0 

2400  5.0  35.7  5.1  8.8  50.4  0  0  0  0 

3000  3.4  34.9  0.1  9.3  55.7  0  0  0  0 

Reaction conditions: atmospheric pressure, 513 K, cat.: 1 mL, 15 min, nO2:nDME = 1:1. 

2.4. Effects of Reaction Time on the Performance of 5%Re‐30%PW12/CNTs 

The effects of reaction time on the conversion of DME and the selectivities of the main products 

over  the  5%Re‐30%PW12/CNTs  catalyst were  investigated. As  can  be  seen  in  Figure  1,  the  total 

selectivity of DMM and DMM2 reaches 59.0% at 15 min. There is a slight decrease from 59.0% to 49.3% 

in the selectivity of DMM and DMM2, and DME conversion has no obvious changes during the 300‐

min reaction. The 5%Re‐30%PW12/CNTs catalyst exhibits high initial activity. 

 

Figure 1. Effects of reaction time on the performance of 5%Re‐30%PW12/CNTs. 

2.5. Catalyst Characterization 

2.5.1. XRD 

Figure 2 shows the XRD patterns of CNT‐supported Re catalysts with different PW12 content. 

For the Re/CNTs catalyst, only diffraction peaks of CNTs exist and no peaks of Re oxides are found, 

indicating that Re oxides are highly dispersed on the catalyst surface. When 20%PW12 is introduced 

0 50 100 150 200 250 3000

10

20

30

40

50

60

HCHO

Co

nve

rsio

n/S

ele

ctiv

ity(%

)

Reaction time/min

DMM

DMM2

DME

CH3OH

Figure 1. Effects of reaction time on the performance of 5%Re-30%PW12/CNTs.

2.5. Catalyst Characterization

2.5.1. XRD

Figure 2 shows the XRD patterns of CNT-supported Re catalysts with different PW12 content.For the Re/CNTs catalyst, only diffraction peaks of CNTs exist and no peaks of Re oxides are found,indicating that Re oxides are highly dispersed on the catalyst surface. When 20%PW12 is introduced to

Catalysts 2016, 6, 43 5 of 12

Re/CNTs, the diffraction peaks of PW12 appear, and the intensity of the peak becomes stronger, whilethe diffraction peaks of CNTs become weaker with the increase of PW12 content.

Catalysts 2016, 6, 43  5 of 12 

to Re/CNTs, the diffraction peaks of PW12 appear, and the intensity of the peak becomes stronger, 

while the diffraction peaks of CNTs become weaker with the increase of PW12 content. 

 

Figure 2. XRD profiles of 5%Re‐PW12/CNTs with different PW12 content. 

2.5.2. BET Surface Area 

Table 4 shows the textural properties of CNT‐supported catalysts. 5%Re/CNTs have much larger 

surface area than other catalysts. 30%PW12 introduction decreases the BET surface area of 5%Re/CNTs 

from 217.4 to 90.9 m2∙g−1 and leads to a decrease in the pore volume. It appears more obvious that the 

BET surface area of the catalyst decreases to 15.2 m2∙g−1 sharply, and the pore volume is only as low 

as 0.062 cm3∙g−1 when 80% PW12 is introduced to Re/CNTs. This may be due to the pore blockage and 

the surface coverage by excessive PW12.  It can be seen  in Table 4  that  the BET surface area of  the 

5%Re‐30%PW12/CNTs catalyst decreases from 90.9 to 81.1 m2∙g−1 6 h post‐reaction. 

Table 4. Textural properties of the catalysts. 

Catalysts BET Surface Area Total Pore Volume Average Pore Diameter 

A (m2∙g−1)  v (cm3∙g−1)  d (nm) 

5%Re/CNTs  217.4  1.054  19.399 

5%Re‐30%PW12/CNTs  90.9  0.467  20.539 

5%Re‐30%PW12/CNTs   

after reaction 81.1  0.523  25.795 

5%Re‐80%PW12/CNTs  15.2  0.062  16.402 

2.5.3. NH3‐TPD 

Figure 3 shows the NH3‐TPD profiles of 5%Re‐PW12/CNTs with different PW12 content. Re/CNTs 

only has weak acid sites due to an NH3 desorption peak at about 430 K. When PW12 was introduced 

into Re/CNTs, two NH3 desorption peaks at about 470 and 630 K, corresponding to weak acid sites 

and strong acid sites, appeared respectively. In order to compare the changes of acid sites after PW12 

introduction,  the  area  of NH3 desorption peaks were  integrated  (see Table  5). By  increasing  the 

content of PW12, the number of the weak acid sites and the strong acid sites becomes larger. The ratio 

of S1(weak acid sites)/S2(strong acid sites)  is highest when PW12 content  is 30%. According to the 

reaction results, the increased amount of weak acid sites can favor the formation of DMM and DMM2 

from DME oxidation. 

10 20 30 40 50 60 70

0

0000

0000

0000

d

c

b

▼

★★

★

▼▼▼▼▼

▼▼▼▼

▼ - PW12

★ - CNTs

a-5%Re/CNTs; b-5%Re-20%PW12

/CNTs;

c-5%Re-30%PW12

/CNTs; d-5%Re-40%PW12

/CNTs

Inte

nsity

(a.u

.)

2Theta/degree

a

Figure 2. XRD profiles of 5%Re-PW12/CNTs with different PW12 content.

2.5.2. BET Surface Area

Table 4 shows the textural properties of CNT-supported catalysts. 5%Re/CNTs have much largersurface area than other catalysts. 30%PW12 introduction decreases the BET surface area of 5%Re/CNTsfrom 217.4 to 90.9 m2¨ g´1 and leads to a decrease in the pore volume. It appears more obvious that theBET surface area of the catalyst decreases to 15.2 m2¨ g´1 sharply, and the pore volume is only as lowas 0.062 cm3¨ g´1 when 80% PW12 is introduced to Re/CNTs. This may be due to the pore blockageand the surface coverage by excessive PW12. It can be seen in Table 4 that the BET surface area of the5%Re-30%PW12/CNTs catalyst decreases from 90.9 to 81.1 m2¨ g´1 6 h post-reaction.

Table 4. Textural properties of the catalysts.

Catalysts BET Surface Area Total Pore Volume Average Pore DiameterA (m2¨ g´1) v (cm3¨ g´1) d (nm)

5%Re/CNTs 217.4 1.054 19.3995%Re-30%PW12/CNTs 90.9 0.467 20.5395%Re-30%PW12/CNTs

after reaction 81.1 0.523 25.795

5%Re-80%PW12/CNTs 15.2 0.062 16.402

2.5.3. NH3-TPD

Figure 3 shows the NH3-TPD profiles of 5%Re-PW12/CNTs with different PW12 content. Re/CNTsonly has weak acid sites due to an NH3 desorption peak at about 430 K. When PW12 was introducedinto Re/CNTs, two NH3 desorption peaks at about 470 and 630 K, corresponding to weak acid sitesand strong acid sites, appeared respectively. In order to compare the changes of acid sites after PW12

introduction, the area of NH3 desorption peaks were integrated (see Table 5). By increasing the contentof PW12, the number of the weak acid sites and the strong acid sites becomes larger. The ratio ofS1(weak acid sites)/S2(strong acid sites) is highest when PW12 content is 30%. According to thereaction results, the increased amount of weak acid sites can favor the formation of DMM and DMM2

from DME oxidation.

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Figure 3. NH3‐TPD profiles of 5%Re‐PW12/CNTs with different PW12 content. 

Table 5. Results of NH3‐TPD integration. 

Catalysts Weak Acid Sites

Area(S1) 

Strong Acid Sites

Area(S2) 

Ratio 

S1/S2 

5%Re/CNTs  100  ‐  ‐ 

5%Re‐20%PW12/CNTs  89.8  10.2  8.8 

5%Re‐30%PW12/CNTs  90.0  10.0  9.0 

5%Re‐40%PW12/CNTs  86.9  13.1  6.6 

2.5.4. H2‐TPR 

Figure  4  shows H2‐TPR  profiles  of  5%Re‐PW12/CNTs with  different  PW12  content.  For  the 

Re/CNTs catalyst, the peaks for H2 consumption appear at about 638 K. An evident shift to  lower 

temperature is observed after the introduction of PW12 into Re/CNTs, suggesting that the addition of 

PW12 can significantly facilitate the reduction of Re oxide species. When the PW12 content is 30%, the 

temperature  of  reduction  peak  reaches  its  lowest  value  at  509  K,  which  suggests  that  5%Re‐

30%PW12/CNTs  exhibit  strong  redox  ability.  The  interaction  of  PW12  and  the  surface  Re  species 

increases the reducibility of Re‐PW12/CNTs, consistent with the results of the introduction of PO43− 

and SO42−, affecting the reducibility of VOx/TS‐1 [29]. 

 

Figure 4. H2‐TPR profiles of 5%Re‐PW12/CNTs with different PW12 content. 

400 500 600 700

-1000

-500

0

500

Inte

nsi

ty(a

.u.)

Temperature/K

a-5%Re/CNT; b-5%Re-20%PW12

/CNT;

c-5%Re-30%PW12

/CNT;d-5%Re-40%PW12

/CNT.

a

b

c

d

400 500 600 700

0

2000

4000

Inte

nsi

ty(a

.u)

Temperature/K

a-5%Re/CNTs; b-5%Re-20%PW12

/CNTs;

c-5%Re-30%PW12

/CNTs;d-5%Re-40%PW12

/CNTs.

a

b

c

d

Figure 3. NH3-TPD profiles of 5%Re-PW12/CNTs with different PW12 content.

Table 5. Results of NH3-TPD integration.

Catalysts Weak Acid SitesArea(S1)

Strong Acid SitesArea(S2)

RatioS1/S2

5%Re/CNTs 100 - -5%Re-20%PW12/CNTs 89.8 10.2 8.85%Re-30%PW12/CNTs 90.0 10.0 9.05%Re-40%PW12/CNTs 86.9 13.1 6.6

2.5.4. H2-TPR

Figure 4 shows H2-TPR profiles of 5%Re-PW12/CNTs with different PW12 content. For the Re/CNTscatalyst, the peaks for H2 consumption appear at about 638 K. An evident shift to lower temperatureis observed after the introduction of PW12 into Re/CNTs, suggesting that the addition of PW12 cansignificantly facilitate the reduction of Re oxide species. When the PW12 content is 30%, the temperatureof reduction peak reaches its lowest value at 509 K, which suggests that 5%Re-30%PW12/CNTs exhibitstrong redox ability. The interaction of PW12 and the surface Re species increases the reducibilityof Re-PW12/CNTs, consistent with the results of the introduction of PO4

3´ and SO42´, affecting the

reducibility of VOx/TS-1 [29].

Catalysts 2016, 6, 43  6 of 12 

 

Figure 3. NH3‐TPD profiles of 5%Re‐PW12/CNTs with different PW12 content. 

Table 5. Results of NH3‐TPD integration. 

Catalysts Weak Acid Sites

Area(S1) 

Strong Acid Sites

Area(S2) 

Ratio 

S1/S2 

5%Re/CNTs  100  ‐  ‐ 

5%Re‐20%PW12/CNTs  89.8  10.2  8.8 

5%Re‐30%PW12/CNTs  90.0  10.0  9.0 

5%Re‐40%PW12/CNTs  86.9  13.1  6.6 

2.5.4. H2‐TPR 

Figure  4  shows H2‐TPR  profiles  of  5%Re‐PW12/CNTs with  different  PW12  content.  For  the 

Re/CNTs catalyst, the peaks for H2 consumption appear at about 638 K. An evident shift to  lower 

temperature is observed after the introduction of PW12 into Re/CNTs, suggesting that the addition of 

PW12 can significantly facilitate the reduction of Re oxide species. When the PW12 content is 30%, the 

temperature  of  reduction  peak  reaches  its  lowest  value  at  509  K,  which  suggests  that  5%Re‐

30%PW12/CNTs  exhibit  strong  redox  ability.  The  interaction  of  PW12  and  the  surface  Re  species 

increases the reducibility of Re‐PW12/CNTs, consistent with the results of the introduction of PO43− 

and SO42−, affecting the reducibility of VOx/TS‐1 [29]. 

 

Figure 4. H2‐TPR profiles of 5%Re‐PW12/CNTs with different PW12 content. 

400 500 600 700

-1000

-500

0

500

Inte

nsi

ty(a

.u.)

Temperature/K

a-5%Re/CNT; b-5%Re-20%PW12

/CNT;

c-5%Re-30%PW12

/CNT;d-5%Re-40%PW12

/CNT.

a

b

c

d

400 500 600 700

0

2000

4000

Inte

nsi

ty(a

.u)

Temperature/K

a-5%Re/CNTs; b-5%Re-20%PW12

/CNTs;

c-5%Re-30%PW12

/CNTs;d-5%Re-40%PW12

/CNTs.

a

b

c

d

Figure 4. H2-TPR profiles of 5%Re-PW12/CNTs with different PW12 content.

Catalysts 2016, 6, 43 7 of 12

2.5.5. XPS

Figure 5 shows the Re 4f and O1s XPS spectra of the 5%Re/CNTs and 5%Re-30%PW12/CNTscatalysts. In contrast, a peak at 45.6 eV is found over the 5%Re/CNTs catalyst, which should ascribe tothe Re 4f7/2 level for Re7+ species, indicating that Re7+ species were mainly present on the surface ofRe/CNTs [20,23]. However, in our previous XPS study, Re7+ was observed at 46.6eV over 5%Re/TiO2.This also indicates that the existence form of Re species changes due to the different interaction of Reand support. However, when PW12 is introduced to Re/CNTs, the most remarkable change is theappearance of a peak at 41.9 eV assigned to Re4+ species [20,23]. This suggests a strong interactionbetween Re oxide species and the surface of CNTs in line with the results of TPR, which indicates thatthe introduction of PW12 evidently changes the surface properties of CNTs and further facilitates theformation of Re4+ species. Additionally, the intensity of O1s has an obvious change before and afterPW12 introduction. This further proves that the species of Re oxides are changed by PW12. Combinedwith the reaction results, the presence of both Re4+ and Re7+ species are further proved to promote theformation of DMM and DMM2.

Catalysts 2016, 6, 43  7 of 12 

2.5.5. XPS 

Figure 5 shows  the Re 4f and O1s XPS spectra of  the 5%Re/CNTs and 5%Re‐30%PW12/CNTs 

catalysts. In contrast, a peak at 45.6 eV is found over the 5%Re/CNTs catalyst, which should ascribe 

to the Re 4f7/2 level for Re7+ species, indicating that Re7+ species were mainly present on the surface of 

Re/CNTs [20,23]. However, in our previous XPS study, Re7+ was observed at 46.6eV over 5%Re/TiO2. 

This also indicates that the existence form of Re species changes due to the different interaction of Re 

and support. However, when PW12  is  introduced  to Re/CNTs,  the most remarkable change  is  the 

appearance of a peak at 41.9 eV assigned to Re4+ species [20,23]. This suggests a strong interaction 

between Re oxide species and the surface of CNTs in line with the results of TPR, which indicates 

that the introduction of PW12 evidently changes the surface properties of CNTs and further facilitates 

the formation of Re4+ species. Additionally, the intensity of O1s has an obvious change before and 

after PW12  introduction. This  further proves  that  the  species  of Re  oxides  are  changed  by PW12. 

Combined with the reaction results, the presence of both Re4+ and Re7+ species are further proved to 

promote the formation of DMM and DMM2. 

(a)  (b) 

Figure 5. Re 4f (a) and O1s; (b) XPS spectra for 5%Re/CNTs and 5%Re‐30%PW12/CNTs. 

2.5.6. TEM 

Figure 6 demonstrates the TEM images of 5%Re/CNTs, 5%Re‐30%PW12/CNTs before and after 

the catalytic oxidation of DME at 513 K and 5%Re‐80%PW12/CNTs. Figure 6 conveys that there are 

distinctive differences in the images between 5%Re/CNTs and 5%Re‐30%PW12/CNTs catalysts. Over 

5%Re/CNTs, no ReOx is found due to highly dispersed Re species over the surface of CNTs. It can be 

seen that the inner pores of CNTs are filled with PW12 after 30%PW12 introduction, and, especially, 

ReOx particles are clearly found over the outer surface of CNTs and the particle size of ReOx is about 

0.7 nm. After reaction, the sample shows some agglomeration of PW12 and Re species. When PW12 

content is increased to 80%, excessive PW12 clearly deposits not only on the inner surface, but also the 

outer surface of CNTs. 

 

50 45 40

0

000

b

Inte

nsi

ty(a

.u.)

Binding energy/eV

a-5%Re/CNTs; b-5%Re-30%PW12

/CNTs a

a

525 530 535 5400

5000

0000

5000

b

a-5%Re/CNTs; b-5%Re-30%PW12

/CNTs

Inte

nsity

(a.u

.)

Binding energy/eV

a

b

Figure 5. Re 4f (a) and O1s; (b) XPS spectra for 5%Re/CNTs and 5%Re-30%PW12/CNTs.

2.5.6. TEM

Figure 6 demonstrates the TEM images of 5%Re/CNTs, 5%Re-30%PW12/CNTs before and afterthe catalytic oxidation of DME at 513 K and 5%Re-80%PW12/CNTs. Figure 6 conveys that thereare distinctive differences in the images between 5%Re/CNTs and 5%Re-30%PW12/CNTs catalysts.Over 5%Re/CNTs, no ReOx is found due to highly dispersed Re species over the surface of CNTs.It can be seen that the inner pores of CNTs are filled with PW12 after 30%PW12 introduction, and,especially, ReOx particles are clearly found over the outer surface of CNTs and the particle size ofReOx is about 0.7 nm. After reaction, the sample shows some agglomeration of PW12 and Re species.When PW12 content is increased to 80%, excessive PW12 clearly deposits not only on the inner surface,but also the outer surface of CNTs.

Catalysts 2016, 6, 43  7 of 12 

2.5.5. XPS 

Figure 5 shows  the Re 4f and O1s XPS spectra of  the 5%Re/CNTs and 5%Re‐30%PW12/CNTs 

catalysts. In contrast, a peak at 45.6 eV is found over the 5%Re/CNTs catalyst, which should ascribe 

to the Re 4f7/2 level for Re7+ species, indicating that Re7+ species were mainly present on the surface of 

Re/CNTs [20,23]. However, in our previous XPS study, Re7+ was observed at 46.6eV over 5%Re/TiO2. 

This also indicates that the existence form of Re species changes due to the different interaction of Re 

and support. However, when PW12  is  introduced  to Re/CNTs,  the most remarkable change  is  the 

appearance of a peak at 41.9 eV assigned to Re4+ species [20,23]. This suggests a strong interaction 

between Re oxide species and the surface of CNTs in line with the results of TPR, which indicates 

that the introduction of PW12 evidently changes the surface properties of CNTs and further facilitates 

the formation of Re4+ species. Additionally, the intensity of O1s has an obvious change before and 

after PW12  introduction. This  further proves  that  the  species  of Re  oxides  are  changed  by PW12. 

Combined with the reaction results, the presence of both Re4+ and Re7+ species are further proved to 

promote the formation of DMM and DMM2. 

(a)  (b) 

Figure 5. Re 4f (a) and O1s; (b) XPS spectra for 5%Re/CNTs and 5%Re‐30%PW12/CNTs. 

2.5.6. TEM 

Figure 6 demonstrates the TEM images of 5%Re/CNTs, 5%Re‐30%PW12/CNTs before and after 

the catalytic oxidation of DME at 513 K and 5%Re‐80%PW12/CNTs. Figure 6 conveys that there are 

distinctive differences in the images between 5%Re/CNTs and 5%Re‐30%PW12/CNTs catalysts. Over 

5%Re/CNTs, no ReOx is found due to highly dispersed Re species over the surface of CNTs. It can be 

seen that the inner pores of CNTs are filled with PW12 after 30%PW12 introduction, and, especially, 

ReOx particles are clearly found over the outer surface of CNTs and the particle size of ReOx is about 

0.7 nm. After reaction, the sample shows some agglomeration of PW12 and Re species. When PW12 

content is increased to 80%, excessive PW12 clearly deposits not only on the inner surface, but also the 

outer surface of CNTs. 

 

50 45 40

0

000

b

Inte

nsi

ty(a

.u.)

Binding energy/eV

a-5%Re/CNTs; b-5%Re-30%PW12

/CNTs a

a

525 530 535 5400

5000

0000

5000

b

a-5%Re/CNTs; b-5%Re-30%PW12

/CNTs

Inte

nsity

(a.u

.)

Binding energy/eV

a

b

Figure 6. Cont.

Catalysts 2016, 6, 43 8 of 12Catalysts 2016, 6, 43  8 of 12 

 

Figure 6. TEM images of 5%Re/CNTs (a); 5%Re‐30%PW12/CNTs (b); 5%Re‐30%PW12/CNTs 6 h post‐

reaction (c) and 5%Re‐80%PW12/CNTs (d). 

2.5.7. SEM 

The SEM micrographs of 5%Re/CNTs, 5%Re‐30%PW12/CNTs, 5%Re‐30%PW12/CNTs 6 h post‐

reaction and 5%Re‐80%PW12/CNTs catalysts are demonstrated in Figure 7. It can be seen in Figure 7 that 

PW12 also exists over the outer surface of CNTs (TEM has proved that PW12 can enter the inner surface 

of  CNTs)  after  30%PW12  introduction  to  Re/CNTs. After  reaction,  the  catalyst  particles  tend  to 

aggregate,  leading  to  the  decrease  of  BET  surface  area  (Table  4).  In  evidence,  PW12  and  CNTs 

agglomerated almost completely due  to  the excessive  introduction of PW12  (80%PW12). Combined 

with the TEM results, PW12 prefers to enter the inner pores of CNTs when PW12 content is low, while 

PW12 mainly deposits on the outer surface of CNTs along with the increase of PW12 content. 

 

Figure 7. SEM images of 5%Re/CNTs (a); 5%Re‐30%PW12/CNTs (b); 5%Re‐30%PW12/CNTs 6 h post‐

reaction (c) and 5%Re‐80%PW12/CNTs (d). 

Figure 6. TEM images of 5%Re/CNTs (a); 5%Re-30%PW12/CNTs (b); 5%Re-30%PW12/CNTs 6 hpost-reaction (c) and 5%Re-80%PW12/CNTs (d).

2.5.7. SEM

The SEM micrographs of 5%Re/CNTs, 5%Re-30%PW12/CNTs, 5%Re-30%PW12/CNTs 6 hpost-reaction and 5%Re-80%PW12/CNTs catalysts are demonstrated in Figure 7. It can be seenin Figure 7 that PW12 also exists over the outer surface of CNTs (TEM has proved that PW12 canenter the inner surface of CNTs) after 30%PW12 introduction to Re/CNTs. After reaction, the catalystparticles tend to aggregate, leading to the decrease of BET surface area (Table 4). In evidence, PW12

and CNTs agglomerated almost completely due to the excessive introduction of PW12 (80%PW12).Combined with the TEM results, PW12 prefers to enter the inner pores of CNTs when PW12 content islow, while PW12 mainly deposits on the outer surface of CNTs along with the increase of PW12 content.

Catalysts 2016, 6, 43  8 of 12 

 

Figure 6. TEM images of 5%Re/CNTs (a); 5%Re‐30%PW12/CNTs (b); 5%Re‐30%PW12/CNTs 6 h post‐

reaction (c) and 5%Re‐80%PW12/CNTs (d). 

2.5.7. SEM 

The SEM micrographs of 5%Re/CNTs, 5%Re‐30%PW12/CNTs, 5%Re‐30%PW12/CNTs 6 h post‐

reaction and 5%Re‐80%PW12/CNTs catalysts are demonstrated in Figure 7. It can be seen in Figure 7 that 

PW12 also exists over the outer surface of CNTs (TEM has proved that PW12 can enter the inner surface 

of  CNTs)  after  30%PW12  introduction  to  Re/CNTs. After  reaction,  the  catalyst  particles  tend  to 

aggregate,  leading  to  the  decrease  of  BET  surface  area  (Table  4).  In  evidence,  PW12  and  CNTs 

agglomerated almost completely due  to  the excessive  introduction of PW12  (80%PW12). Combined 

with the TEM results, PW12 prefers to enter the inner pores of CNTs when PW12 content is low, while 

PW12 mainly deposits on the outer surface of CNTs along with the increase of PW12 content. 

 

Figure 7. SEM images of 5%Re/CNTs (a); 5%Re‐30%PW12/CNTs (b); 5%Re‐30%PW12/CNTs 6 h post‐

reaction (c) and 5%Re‐80%PW12/CNTs (d). Figure 7. SEM images of 5%Re/CNTs (a); 5%Re-30%PW12/CNTs (b); 5%Re-30%PW12/CNTs 6 hpost-reaction (c) and 5%Re-80%PW12/CNTs (d).

For the 5%Re/CNTs catalyst, ReOx is dispersed uniformly on the surface of CNTs according tothe results of XRD and TEM. According to the results of XPS, Re species mainly exists in the form of

Catalysts 2016, 6, 43 9 of 12

Re2O7. After PW12 introduction to Re/CNTs, PW12 dispersed on the inner and outer surface of CNTsand affected the dispersion of ReOx on the surface of CNTs. The introduction of optimum amount ofPW12 not only increased the total amount of acid sites, but also significantly changed the oxidationstate of Re species and facilitated the formation of Re4+ species. The characterization of the catalystsbefore and after reaction indicated that the agglomeration of active species may be the main reasons ofthe catalyst deactivation.

Due to the high stability of the DME molecule, the activation of DME molecule at lowertemperature is very difficult, while a higher temperature easily leads to the bond-breaking of C–O andC–H concurrently, and further results in the complex products along with COx production. Therefore,how to activate DME at a lower temperature and selectively convert DME to target chemicals withoutCOx formation is a challenging task. DMMx selectivity is a very important factor for the DME highlyselective oxidation to a diesel oil additive. The higher DMMx selectivity is, the better DME utilizationis, provided that no COx is produced during DME oxidation reactions. As the main products, DMMx,HCHO and CH3OH can be separated by distillation according to their different boiling points. In thepresent work, the low-temperature oxidation of DME to DMMx with high DMMx selectivity of 59.0%and DME conversion of 8.9% was realized with no COx production over the 5%Re-30%PW12/CNTs.We should manage to increase DMMx selectivity in our future work based on previous results. Theconversion of DME is not too high, but increasing the DME conversion easily leads to more by-products.For the 5%Re-30%PW12/CNTs catalyst, enhancing the PW12 content, increasing reaction temperatureand decreasing GHSV can raise the DME conversion; however, DMMx selectivity decreases clearly, andCOx is also produced. Combining the reaction results, the 5%Re-30%PW12/CNTs catalyst has someadvantages if it can be used in the related field in the future. The reaction temperature is 80 K lower thanthat reported in our previous work; more importantly, no COx was formed over 5%Re-30%PW12/CNTs.Additionally, the catalyst stability was also increased, compared to the catalyst in the previous work.Though the once-through DME conversion is not very high, the unconverted DME can be recycledto improve its utilization. These promising results can help us thoroughly understand the reactionmechanism of DME activation and offer further possible industrial applications in the future.

3. Experimental Section

3.1. Catalyst Preparation

H3PW12O40-modified Re/CNTs catalysts were prepared by the incipient wetness impregnationmethod. An aqueous solution of H3PW12O40 (Shanghai Chemical Co., Shanghai, China) wasimpregnated in CNTs ((Multi-walled carbon nanotubes, inner diameter = 4–8 nm, outer diameter<10–20 nm, Chengdu Organic Chemicals Co. Ltd., Chengdu, China). Raw CNTs were refluxed in HNO3

(68 wt. %) for 14 h at 140 ˝C in an oil bath; then the mixture was filtered and washed with deionizedwater, followed by drying at 60 ˝C for 12 h.) at 298 K for 6 h, then dried overnight at 393 K, andcalcined at 673 K for 4 h. An aqueous solution of ammonium perrhenate (NH4ReO4, Strem Chemicals,Inc., Newburyport, MA, USA) was used to impregnate the H3PW12O40/CNTs, and the followingprocedures were the same as the above. The catalyst was designated as 5%Re-20%PW12/CNTs,5%Re-30%PW12/CNTs, and 5%Re-40%PW12/CNTs. Re/CNTs was prepared according to the aboveprocedures. For the catalysts used in this study, Re and PW12 refer to Re2O7 and H3PW12O40,respectively. The amount of Re in the catalyst refers to the amount of Re2O7.

3.2. Catalytic Oxidation of DME

The catalytic oxidation of DME was carried out in a continuous flow type fixed-bed reactor.The catalyst (1 mL, 20–40 mesh) was diluted with ground quartz to prevent the over-heating of thecatalyst due to exothermic reaction. The catalyst was treated in flow of O2 (30 mL/min) for 1 h beforereaction. The reactant mixture consisted of DME and O2 with ratio of nO2:nDME = 1:1. The reactionproducts were analyzed by gas chromatography GC-2014CPF/SPL (Shimadzu Co., Kyoto, Japan)

Catalysts 2016, 6, 43 10 of 12

equipped with a flame ionization detector (60 m ˆ 0.25 mm, DB-1 column, Agilent Technologies Inc.,Palo Alto, IA, USA) and GC-2014 (Shimadzu Co., Kyoto, Japan) with a thermal conductivity detector(Porapak T column, Waters Corporation, Milford, MA, USA). GC-4000A (TDX-01 column, East & WestAnalytical Instruments, Inc., Beijing, China) with thermal conductivity detectors was used to analyzeH2, CO, CO2 and CH4.

The data of the whole work was calculated based on carbon balance, and the carbon balances ofmost experiments were within 95%–99%.

3.3. Structure and Properties Characterization

3.3.1. BET Surface Area

Surface areas of the samples were measured by a BET nitrogen adsorption method at 77.35 Kusing a TriStar 300 machine (Micromeritics, Atlanta, GA, USA). The samples were treated at 473 Kunder vacuum conditions for 8 h before BET test.

3.3.2. X-ray Diffraction (XRD)

XRD patterns were measured on a Bruker Advanced X-Ray Solutions/D8-Advance (Bruker,Karlsruhe, Germany) using Cu Ka radiation. The anode was operated at 40 kV and 40 mA. The 2-Thetaangles were scanned from 5˝ to 70˝.

3.3.3. Temperature Programmed Desorption (TPD)

The NH3-TPD profiles were obtained in a fixed-bed reactor system connected with a thermalconductivity detector (Tianjin Xianquan Co. Ltd., Tianjin, China). The catalyst sample (100 mg) waspretreated at 673 K under N2 flow (40 mL/min) for 2 h and then cooled down to 373 K under N2 flow.Then NH3 of 40 mL/min was introduced into the flow system for a continuous 20 min before doingTPD. The dose amount of NH3 maintained the same for all the samples investigated. The TPD profileswere recorded at a temperature rising rate of 5 K/min from 373 to 923 K.

3.3.4. Temperature Programmed Reduction (TPR)

H2-TPR was conducted in a fixed-bed reactor system equipped with a thermal conductivitydetector (Tianjin Xianquan Co. Ltd., Tianjin, China). The sample (100 mg) was pretreated in Ar at673 K for 0.5 h and then cooled down to 323 K. After that, a 10%H2/Ar mixed gas was switched onand the temperature was increased linearly at a rate of 5 K/min from 323 K to 923 K.

3.3.5. X-ray Photoelectron Spectra (XPS)

XPS were measured on a XPS-AXIS Ultra of Kratos Co. (Manchester, UK) by using Mg Karadiation (Hν = 1253.6 eV) with X-ray power of 225 W (15 kV, 15 mA).

3.3.6. Transmission Electron Microscope (TEM)

TEM images were taken on a JEM-2010 Transmission electron microscope (JEOL Company,Tokyo, Japan).

3.3.7. Scanning Electron Microscope (SEM)

SEM images were taken on a JSM-35C scanning electron microscope (JEOL Company, Tokyo,Japan) operated at 25 kV.

4. Conclusions

Low-temperature oxidation of DME to DMM and DMM2 was successfully realized overa CNT-supported Re-based catalyst. The introduction of PW12 markedly increases the activity of

Catalysts 2016, 6, 43 11 of 12

Re/CNTs. The total selectivity of DMM and DMM2 reaches 59.0% with DME conversion of 8.9% at513 K without the formation of COx over 5%Re-30%PW12/CNTs. CNTs as support play an importantrole in promoting the synthesis of DMMx and inhibiting the formation of COx due to its uniquephysical and chemical properties.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. 21373253,No. 20903114) and Youth Innovation Promotion Association CAS (No. 2014155).

Author Contributions: Qingde Zhang performed experiments and wrote the paper; Wenfeng Wang andZhenzhou Zhang characterized the catalysts; Yisheng Tan and Yizhuo Han conceived the experiments andrevised the paper.

Conflicts of Interest: The authors declare no conflict of interest.

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